TWI711634B - Piezoelectric film, method of manufacturing piezoelectric film, and piezoelectric element - Google Patents

Abstract

Problem The present invention provides a piezoelectric thin film, a method for manufacturing a piezoelectric thin film, and a piezoelectric element that can suppress the decrease in the piezoelectric constant d31. Solution The piezoelectric film of the present invention contains a fluororesin as the piezoelectric material, the fluororesin includes a repeating unit derived from vinylidene fluoride as the main structural unit, and the piezoelectric constant d31 of the piezoelectric film is 20 pC/ N or more, and the shrinkage start extrapolation start temperature determined by TMA measurement is 90°C or more and 115°C or less. The difference in piezoelectric constant d31 measured before and after heating the piezoelectric thin film at 100° C. for 24 hours is 20% or less with respect to the piezoelectric constant d31 before heating 24 hours.

Description

Piezoelectric film, method of manufacturing piezoelectric film, and piezoelectric element

The present invention relates to a piezoelectric thin film with a fluororesin as a piezoelectric material, a method for manufacturing a piezoelectric thin film, and a piezoelectric element with a piezoelectric thin film. The fluororesin includes repeats derived from vinylidene fluoride Unit as the main structural unit.

Piezoelectric films with polyvinylidene fluoride as the piezoelectric material are used in various sensors such as vibration sensors, touch sensors, ultrasonic sensors, acceleration sensors, etc., vibration brakes, and brakes. Various types of brakes such as vibration brakes, etc. In Patent Document 1, when a polyvinylidene fluoride film is produced, the uniaxial stretching process of the polyvinylidene fluoride sheet in the MD (Machine Direction) direction and the polyvinylidene fluoride sheet are simultaneously performed Polarization treatment. The piezoelectric thin film manufactured by extending in the MD direction is excellent in terms of having a high voltage electric constant d31. Prior technical literature Patent literature

Patent Document 1: Japanese Patent Laid-Open No. 2008-171935

The problem to be solved by the invention In addition, the piezoelectric film manufactured by stretching is usually temporarily stored in the form of a rolled product, and then used for the processing of sensors and brakes. At this time, the piezoelectric film is drawn out from the rolled product, and the piezoelectric film is heated at a high temperature during the processing of the sensor, brake, etc., which is accompanied by heat treatment such as vapor deposition and lamination. As a result, the piezoelectric constant d31 of the piezoelectric thin film is a new problem. In addition, a piezoelectric element having such a piezoelectric thin film with a reduced piezoelectric constant d31 may cause problems.

The object of the present invention is to provide a piezoelectric thin film, a method of manufacturing a piezoelectric thin film, and a piezoelectric element that can suppress the decrease in the piezoelectric constant d31. Technical means to solve the problem

The piezoelectric film used to solve the above-mentioned problems is a piezoelectric film containing a fluororesin as a piezoelectric material. The fluororesin contains a repeating unit derived from vinylidene fluoride as the main structural unit, and the pressure of the piezoelectric film is The electric constant d31 is 20 pC/N or more, and the extrapolation start temperature of the shrinkage obtained by the TMA measurement is 90°C or more and 135°C or less.

The manufacturing method of piezoelectric thin film to solve the above-mentioned problems includes: thin film forming process, stretching and polarization processing on the sheet formed of fluororesin, thereby forming a piezoelectric crystalline polymer thin film ( Hereinafter, also referred to as "crystalline polymer film"), the aforementioned fluororesin contains repeating units derived from vinylidene fluoride as the main structural unit; the relaxation process, 90°C or more and 115°C or less is the first temperature, and the aforementioned crystal The crystalline polymer film is heated at the first temperature for 5 seconds or more and 130 seconds or less, thereby thermally fixing the crystalline polymer film and alleviating residual strain; and a secondary heating process to reduce the temperature of the crystalline polymer film. The crystalline polymer film is further heated at the first temperature or higher and 140° C. or lower to produce a piezoelectric film.

The method of manufacturing piezoelectric film to solve the above-mentioned problems is a method of manufacturing piezoelectric film from a crystalline polymer sheet, and includes: a film forming process, stretching a sheet formed of a fluororesin, and Chemical treatment to form a piezoelectric crystalline polymer film. The aforementioned fluororesin contains repeating units derived from vinylidene fluoride as the main structural unit; and to ease the process at a temperature above 115°C and below 150°C Next, the crystalline polymer film is heated for 10 seconds or more and 140 seconds or less, thereby thermally fixing the crystalline polymer film and alleviating residual strain.

According to the aforementioned piezoelectric thin film and the aforementioned piezoelectric thin film manufacturing method, even if the piezoelectric thin film is heated during the processing of the piezoelectric thin film, the reduction in piezoelectric constant d31 due to processing can be suppressed.

In the above piezoelectric film, the treatment of heating the piezoelectric film at 100°C for 24 hours is a test treatment. The difference in the piezoelectric constant d31 measured before and after the test treatment is relative to the piezoelectric constant before the test treatment d31 can be 20% or less. According to this piezoelectric film, the attenuation rate of the piezoelectric constant d31 can be ensured to be 20% or less. Therefore, compared with the piezoelectric constant d31 and the extrapolated starting temperature to specify the structure of the piezoelectric film, it can be more reliable Suppress the reduction of piezoelectric constant d31 due to processing.

In the piezoelectric film, the fluororesin may be a homopolymer of vinylidene fluoride. According to this piezoelectric thin film, it is easier to increase the piezoelectric constant d31 of the piezoelectric thin film compared to a structure in which the fluororesin is a copolymer of vinylidene fluoride.

In the manufacturing method of the piezoelectric thin film, the second heating process may heat the crystalline polymer thin film at the first temperature. According to the manufacturing method of the piezoelectric thin film, the heating equipment used in the relaxation process and the heating equipment used in the secondary heating process can also be shared.

The piezoelectric element for solving the above-mentioned problems includes the above-mentioned piezoelectric thin film. Invention effect

According to the piezoelectric thin film, the manufacturing method of the piezoelectric thin film, and the piezoelectric element of the present invention, the reduction of the piezoelectric constant d31 can be suppressed.

Hereinafter, one embodiment of a piezoelectric thin film, a method of manufacturing a piezoelectric thin film, and a piezoelectric element will be described with reference to FIG. 1. In addition, the piezoelectric film is wound into a roll as a product, and is transported. The piezoelectric element with the piezoelectric thin film is manufactured by using the piezoelectric thin film drawn from the product of the piezoelectric thin film. [Piezoelectric film]

The piezoelectric film contains a fluororesin as a piezoelectric material, and the aforementioned fluororesin contains a repeating unit derived from vinylidene fluoride as a main structural unit. The fluororesin is a resin containing a repeating unit derived from vinylidene fluoride as a main structural unit (hereinafter, also referred to as polyvinylidene fluoride resin). In this specification and the like, the "main structural unit" means that when all the repeating units constituting the resin are set to 100 mol%, the vinylidene fluoride-derived repeating unit is 50 mol% or more.

The polyvinylidene fluoride resin is preferably a homopolymer, but may also be a copolymer. The copolymer here is preferably a copolymer of vinylidene fluoride and at least one monomer selected from the group consisting of the following monomers: 1-chloro-1-fluoro-ethylene, 1-chloro-2 -Fluoro-ethylene, trifluoroethylene, chlorotrifluoroethylene, tetrafluoroethylene, tetrafluoropropylene, hexafluoropropylene, and vinyl fluoride.

Furthermore, the inherent viscosity of the polyvinylidene fluoride resin is preferably 1.0 dl/g or more. The intrinsic viscosity is a value measured at 30°C of a dimethylformamide solution with a concentration of 0.4 g/dl. The polyvinylidene fluoride resin with an inherent viscosity of 1.0 dl/g or more can inhibit the shear force acting on the sheet during the necking and stretching, which causes the sheet to break. A polyvinylidene fluoride resin with an inherent viscosity of 1.1 dl/g or more and 2.0 dl/g or less can obtain high strength and good extensibility during necking stretch. The so-called necking extension refers to the extension in a form where the thickness and width of the sheet become thinner (necked part) at a point in the traveling direction of the sheet during the stretching process.

The piezoelectric film may contain components such as resins or additives other than fluororesin. Examples of resins other than fluororesin include methacrylic resin, cellulose derivative resin, polyester resin, and polycarbonate resin.

In addition, as additives, for example, metal oxide particles, coupling agents, and surfactants. The metal oxide particles are, for example, aluminum oxide particles, magnesium oxide particles, zirconium oxide particles, and yttrium oxide particles. The coupling agent has the function of increasing the degree of bonding between the metal oxide particles dispersed in the fluororesin and the piezoelectric material, such as organic titanium compounds, organic silane compounds, organic zirconium compounds, and organic aluminum compounds. Surfactants have the function of improving the affinity between the metal oxide particles dispersed in the fluororesin and the piezoelectric material, such as nonionic surfactants, anionic surfactants, and cationic surfactants.

The piezoelectric thin film satisfies the following [Condition 1] [Condition 2]. [Condition 1] The piezoelectric constant d31 is 20 pC/N or more. [Condition 2] The extrapolation start temperature at the start of shrinkage is 90°C or more and 135°C or less.

Furthermore, the extrapolation start temperature at the start of shrinkage is determined by the TMA (Thermomechanical Analyzer) measurement using the piezoelectric film. Extrapolated onset temperature (Extrapolated onset temperature) is based on the temperature of JIS K0129. The extrapolated starting temperature is the temperature at the intersection of the straight line obtained by extending the reference line on the low temperature side to the high temperature side and the tangent line drawn at the point of the curve on the low temperature side of the wave crest with the largest slope in the heating measurement. The extrapolated starting temperature is the temperature at which the piezoelectric film exhibits thermal shrinkage.

The piezoelectric thin film preferably satisfies the following [Condition 3]. [Condition 3] The attenuation rate of the piezoelectric constant d31 in the test process is 20% or less.

Furthermore, the test treatment is a test in which the piezoelectric film is heated at 100°C for 24 hours. The attenuation rate of the piezoelectric constant d31 is the difference between the piezoelectric constant d31 measured before and after the test treatment relative to the piezoelectric constant before the test treatment d31 ratio.

Furthermore, the piezoelectric thin film may also have surface scratches in the direction consistent with the direction exhibiting the piezoelectric constant d31. Such surface scratches can be obtained by the following method, that is, in the manufacturing process of the piezoelectric film, a conductive heating roller is used for stretching treatment, and the surface of the heating roller becomes rough. The surface scratches of the piezoelectric film can improve the adhesion strength between the piezoelectric film and the electrode. The thickness of the piezoelectric film is, for example, 10 µm or more and 500 µm or less. [Piezoelectric element]

Piezoelectric elements include various sensors such as pressure sensors, vibration sensors, touch sensors, ultrasonic sensors, and acceleration sensors, and various brakes such as brakes for vibration and brakes for vibration damping.

A vibration sensor as an example of a sensor device is provided with a plumb weight, which is displaced by vibration, and the piezoelectric film, which bears the load of the plumb, and has electrodes on the surface and back of the piezoelectric film. In addition, the vibration sensor detects the load change of the plumb weight that is displaced by vibration in the form of a voltage signal between electrodes sandwiching a piezoelectric thin film.

A touch sensor as another example of a sensor device includes a cover film and the above-mentioned piezoelectric film overlapping the cover film, and has electrodes on the surface and back of the piezoelectric film. In addition, the touch sensor detects the voltage signal between the electrodes sandwiching the piezoelectric film, and detects the detected portion of the voltage signal as the portion pressed by the cover film. [The first manufacturing method of piezoelectric film]

The first method of manufacturing a piezoelectric film uses a crystalline polymer sheet as a starting material. The crystalline polymer sheet is formed with fluororesin as the main component. The first manufacturing method of the piezoelectric thin film includes (A) a thin film forming process, (B) a first relaxation process, and (C) a secondary heating process. Between (B) the first relaxation process and (C) the secondary heating process, the crystalline polymer film is wound into a roll shape, for example, it is allowed to stand at room temperature for several days. (A) Thin film formation process

The thin film forming process is a process in which the crystalline polymer sheet is stretched and polarized to form a crystalline polymer film from the crystalline polymer sheet. The extension treatment and the polarization treatment can be performed at the same time, or the polarization treatment can be performed after the extension treatment.

The resin constituting the crystalline polymer sheet is a resin that uses the fluororesin described in the item of the piezoelectric film as the main component. Furthermore, the "main component" here means the component with the largest content among the components used to form the crystalline polymer sheet. The resin constituting the crystalline polymer sheet exhibits piezoelectricity through polarization treatment by applying a DC voltage.

The crystalline polymer sheet before the stretching treatment and the polarization treatment is a sheet that does not have piezoelectricity, and is formed by, for example, a melt extrusion method or a solution casting method. The thickness of the crystalline polymer sheet formed by the melt extrusion method can be appropriately adjusted by changing the stretching conditions. The thickness of the crystalline polymer sheet is, for example, preferably 20 µm or more and 2500 µm or less, and more preferably 40 µm or more and 1500 µm or less. The crystalline polymer sheet with a thickness of 20 µm or more is less likely to break when the crystalline polymer sheet is stretched. The crystalline polymer sheet with a thickness of 2500 µm or less can easily obtain the flexibility of the crystalline polymer sheet suitable for conveying the crystalline polymer sheet during the stretching process.

The crystalline polymer sheet is fed to the conductive heating roller. The crystalline polymer sheet fed to the heating roller takes the extension direction as the MD direction, and is necked and stretched in the contact area with the heating roller. In order to effectively perform necking and stretching, the temperature of the heating roller is set to room temperature or higher and not reaching the melting point of the crystalline polymer sheet, for example, 70°C or higher and 135°C or lower. At this time, apply a DC voltage between the non-contact electrode connected to the DC power supply and the grounded heating roller, thereby simultaneously performing the elongation treatment of the crystalline polymer sheet and the polarization treatment of the crystalline polymer sheet .

In addition, the stretching ratio of the crystalline polymer sheet is, for example, 2.5 times or more and 6.0 times or less. If the stretch ratio of the crystalline polymer sheet is 2.5 times or more, it is easy to stabilize the necking stretch. If the stretch ratio of the crystalline polymer sheet is 6.0 times or less, it is easy to suppress breakage in the crystalline polymer sheet and the crystalline polymer film. (B) The first mitigation process

The first relaxation process is a process in which the crystalline polymer film formed in the film formation process is heated to relax the strain in the crystalline polymer film and heat-fix the crystalline polymer film. Compared with the crystalline polymer film before heat setting, the crystalline polymer film after heat setting is less prone to heat shrinkage during the processing process that is applied with heating treatment. As a method of heating the crystalline polymer film, for example, a method of exposing the crystalline polymer film to hot air, a method of passing the crystalline polymer film through a heating roller, and a combination thereof can be used.

The temperature of the crystalline polymer film in the first relaxation process is the first temperature. The first temperature refers to a temperature above 90°C and below 115°C. The heating time of the crystalline polymer film in the first relaxation process is, for example, 1 minute or more and 5 minutes or less. (C) Secondary heating process

The secondary heating process uses the heat-fixed crystalline polymer film as a heating to improve the heat resistance of the crystalline polymer film. The crystalline polymer film is heated again (reheated) to increase the crystallinity The process of forming a piezoelectric thin film from a molecular film. As a method of reheating the crystalline polymer film, for example, a method of exposing the crystalline polymer film to hot air, a method of passing the crystalline polymer film through a heating roller, and a combination thereof can be used.

The temperature of the crystalline polymer film in the secondary heating process is the second temperature. The second temperature refers to a temperature above the first temperature and below 140°C. Therefore, as the second temperature range, the widest range is 90°C or more and 140°C or less. The heating time of the crystalline polymer film in the secondary heating process is 15 seconds or more and 120 seconds or less, preferably 20 seconds or more and 60 seconds or less. If the second temperature is 140°C or lower, softening of the crystalline polymer film can be suppressed. Furthermore, in the secondary heating process, it is preferable to apply a predetermined tension to the crystalline polymer film while heating at the second temperature. If the heating time is 20 seconds or less, the desired characteristics cannot be obtained, and if it is 120 seconds or more, there are restrictions on the durability of the crystalline polymer film.

Here, as for the piezoelectric film as a product, it is usually wound into a roll or drawn from a roll in order to be suitable for the subsequent processing process of using the piezoelectric film. At this time, the crystalline polymer film and the sensor and other processing processes are extracted and heated in accordance with the surface vapor deposition process, dry lamination process and other processing processes. As a result, even if the crystalline polymer film is heat-fixed, a small amount of heat shrinkage (post-shrinkage) occurs in the crystalline polymer film, which reduces the piezoelectric constant d31 of the crystalline polymer film.

In this respect, in the above-mentioned method of manufacturing a piezoelectric film, the crystalline polymer film wound into a roll is allowed to stand, and then the crystalline polymer film is subjected to a secondary heating process. In addition, the crystalline polymer film rolled into a roll is drawn out in accordance with the secondary heating process, and reheated at a temperature above the first temperature and below 140°C. As a result, the piezoelectric film is pre-corrected for shrinkage during the above-mentioned processing process during the secondary heating process. Furthermore, in the crystalline polymer film after heat setting, the temperature at which the shrinkage starts after the heat setting has already risen. On the other hand, in the secondary heating process, the crystalline polymer film is reheated above the temperature at the time of heat setting. Therefore, even if the temperature of the post-shrinkage starts to rise due to heat fixation, the post-shrinkage can be corrected. [The second manufacturing method of piezoelectric film]

The second manufacturing method of the piezoelectric thin film uses a crystalline polymer sheet as a starting material, as in the first manufacturing method. The second manufacturing method of the piezoelectric thin film includes (A) a thin film forming process, and (B) a second relaxation process. Furthermore, since (A) the thin film forming process is the same as the thin film forming process of the first manufacturing method, the description of the thin film forming process is omitted here. Hereinafter, (B) the second relaxation process will be described in detail. (B) The second mitigation process

The second relaxation process in the second manufacturing method is the same treatment as the first relaxation process in that the crystalline polymer film is heated, but the temperature range of the crystalline polymer film is different. That is, the temperature of the crystalline polymer film in the second relaxation process is preferably higher than 115°C and 150°C or lower, more preferably higher than 130°C and 135°C or lower. Furthermore, the heating time of the crystalline polymer film in the second relaxation process is, for example, preferably 10 seconds or more and 140 seconds or less, more preferably 20 seconds or more and 140 seconds or less.

The temperature in the second relaxation process is not particularly limited as long as it falls within the above-mentioned temperature range. However, if the temperature at which the crystalline polymer film is heated is 115°C or less, a secondary heating process is required as in the first manufacturing method. In addition, if the temperature at which the crystalline polymer film is heated is 150°C or higher, there are restrictions on the durability of the crystalline polymer film.

In this respect, if it is the second manufacturing method that performs the second relaxation process, the (C) secondary heating process performed in the first manufacturing method may be omitted. That is, if it is the second manufacturing method, the (C) secondary heating process may not be performed, and the extrapolation starting temperature may be 90° C. or more and 135° C. or less. In addition, the attenuation rate of the piezoelectric constant d31 of the crystalline polymer film obtained by the second manufacturing method can be reduced. [Examples 1 to 8: First manufacturing method]

A polyvinylidene fluoride (KF#1100 manufactured by KUREHA Co., Ltd.) sheet with a thickness of 100 µm is used as the crystalline polymer sheet, and the crystalline polymer sheet is subjected to necking extension treatment and then polarized deal with. At this time, the crystalline polymer sheet is passed through a heating roller heated to a surface temperature of 120°C, the DC voltage is increased from 0 kV to 20 kV, and polarization treatment is performed after the stretching treatment. Then, the crystalline polymer film of Example with a thickness of 27 µm was obtained.

Next, the first temperature was set to 100°C and the heating time was set to 10 seconds, and the crystalline polymer film of the example was subjected to the first relaxation process using hot air. Furthermore, the first temperature was set to 90°C, the heating time was set to 2 minutes, and the crystalline polymer film exposed to hot air was passed through a heating roller to perform a further first relaxation process. Then, the crystalline polymer film after the first relaxation process using hot air and a heating roller was wound into a roll, and left to stand at room temperature for 3 days. Furthermore, in the first relaxation process using hot air and heating rollers, a difference is set in the feed rate of the crystalline polymer film in the first relaxation process in a way that the tension on the crystalline polymer film is fixed.

Next, the second temperature is set to 90° C. or more and 130° C. or less, and the heating time is set to 20 seconds or more and 60 seconds or less, and the crystalline polymer film after the first relaxation process is subjected to a secondary heating process. Then, any one of the second temperature and the heating time was changed to obtain eight kinds of piezoelectric thin films of the examples. In addition, the second temperature of Example 1 was 90°C, and the heating time was 60 seconds. The second temperature of Example 2 was 100°C, and the heating time was 60 seconds. The second temperature of Example 3 was 110°C, and the heating time was 60 seconds. The second temperature of Example 4 was 120°C, and the heating time was 60 seconds. The second temperature of Example 5 was 125°C, and the heating time was 40 seconds. The second temperature of Example 6 was 125°C, and the heating time was 60 seconds. The second temperature of Example 7 was 130°C, and the heating time was 60 seconds. The second temperature of Example 8 was 130°C, and the heating time was 20 seconds. In addition, except that the secondary heating process was not performed, the same conditions as in the example were used to obtain the piezoelectric thin film of the comparative example. [Examples 9, 10: Second manufacturing method]

Using the crystalline polymer film of the 27 µm-thickness example used in Examples 1 to 8, the heating temperature in the second relaxation process was set to 135°C and the heating time was set to 20 seconds. The polymer film undergoes the second relaxation process using hot air. Furthermore, the temperature was set to 90°C and the heating time was set to 2 minutes, and the crystalline polymer film exposed to hot air was passed through a heating roller to perform a temperature adjustment process at a lower temperature. Then, except that the secondary heating treatment was not performed, the piezoelectric film of Example 9 was obtained in the same manner as in Examples 1 to 8. In addition, the temperature of the heating roller in Example 9 was changed from 90°C to 120°C, the temperature adjustment process was set to the extension of the second relaxation process, and other conditions were set to be the same as in Example 9 to obtain an example 10 piezoelectric film. Furthermore, in Examples 9 and 10, the temperature adjustment process after the second relaxation process suppresses the rapid cooling of the crystalline polymer film immediately after the second relaxation process, and suppresses the wrinkles of the thin crystalline polymer film due to rapid cooling. The thick crystalline polymer film warps due to rapid cooling.

In addition, for the piezoelectric thin films of Examples 1 to 10 and the piezoelectric thin films of Comparative Examples, the extrapolation starting temperature, the piezoelectric constant d31, and the attenuation rate of the piezoelectric constant d31 were measured based on the following conditions. [Audition]

On the front and back surfaces of the piezoelectric thin films of the respective Examples and Comparative Examples, aluminum electrodes with a thickness of 100 nm or more and 800 nm or less were formed by an evaporation method. Next, as a sample for measuring the piezoelectric constant, a test piece with a size of 7 mm×30 mm was cut from the piezoelectric thin film on which the aluminum electrode was formed. In addition, as a sample for extrapolating the initial temperature measurement, a test piece with a size of 3 mm×60 mm was cut from the piezoelectric film. [Measurement of piezoelectric constant d31]

Using the test pieces of the respective examples and comparative examples, the piezoelectric constant d31 was measured under the following conditions. ･Measuring device Rheolograph-Solid (manufactured by Toyo Seiki Seisakusho Co., Ltd.) ･Measuring temperature 23℃ ･Measuring frequency 10 Hz ･Applying tension 1 N [Extrapolation of starting temperature measurement]

Using the test pieces (3 mm×60 mm) of each Example and Comparative Example, the thermal shrinkage behavior was measured under the following conditions, and the extrapolated starting temperature was measured based on the obtained measurement results. ･Measuring device EXSTAR6000 (manufactured by Seiko Instruments Co., Ltd.) ･Starting temperature 30℃ ･Finishing temperature 150℃ ･Rising rate 2℃/min ･Measurement interval 1 second [Measurement of attenuation rate of piezoelectric constant d31]

The test pieces of Examples 1 to 10 and Comparative Examples were subjected to the following test treatments, and the piezoelectric constant d31 before the test and the piezoelectric constant d31 after the test were measured. Then, the attenuation rate of the piezoelectric constant d31 after the test process relative to the piezoelectric constant d31 before the test process is calculated. The attenuation rate is the ratio of the difference between the piezoelectric constant d31 before and after the test process to the piezoelectric constant d31 before the test process. ･Test temperature 100℃ ･Test time 24 hours ･Test device thermostat HT320 (manufactured by Kusumoto Chemical Co., Ltd.)

Types of manufacturing methods in Examples 1 to 10 and Comparative Examples, details of processing conditions in the secondary heating process, details of processing conditions in the second relaxation process, extrapolated starting temperature, piezoelectric constant d31, piezoelectric constant d31 The attenuation rate is shown in Table 1. In addition, the relationship between the extrapolated starting temperature and the attenuation rate of the piezoelectric constant d31 in Examples 1 to 10 and the comparative example is shown in FIG. 1. In addition, in Table 1, the first manufacturing method is described as "(1)", and the second manufacturing method is described as "(2)". In addition, in each of Examples 1 to 8 using the first manufacturing method, as the heat treatment condition, the treatment condition of the secondary heating process is shown, and in the examples 9 and 10 using the second manufacturing method, the heat treatment condition is shown as the heat treatment condition. 2 ease the processing conditions of the manufacturing process. Table 1

As shown in Table 1, it was confirmed that in the piezoelectric film of the comparative example, the piezoelectric constant d31 before the test was 27.9 pC/N, the piezoelectric constant d31 after the test was 21.6 pC/N, and the extrapolation starting temperature was 84.8 ℃, the attenuation rate of piezoelectric constant d31 is 22.6%. In contrast, it was confirmed that the piezoelectric constant d31 after the test was 22.8 pC/N or more and 26.1 pC/N or less in the piezoelectric thin film (Examples 1 to 8) after the secondary heating process, and the starting temperature was extrapolated It is higher than 92.3℃, and the attenuation rate is lower than 17.8%. In addition, it was confirmed that in the piezoelectric film of Example 9 where the heating temperature in the second relaxation process was set to 135°C and the secondary heating process was not performed, the piezoelectric constant d31 after the test was higher than that of Examples 1 to 8. 26.8 pC/N. In the piezoelectric film of Example 10, the piezoelectric constant d31 after the test is 25.3 pC/N, which is higher than that of Examples 1 to 4, 7, and 8. In addition, it was confirmed that the extrapolation starting temperature of the piezoelectric thin films of Examples 9 and 10 was higher than 98.8°C, and the attenuation rate was lower than 7.3%. That is, it was confirmed that Examples 1 to 10 satisfy [Condition 1] [Condition 2] [Condition 3].

In addition, as shown in Table 1 and FIG. 1, in Examples 1 to 10, it was confirmed that the higher the extrapolation starting temperature, the lower the attenuation rate. Furthermore, it is confirmed that the second temperature in the secondary heating process is higher. In addition, the longer the heating time in the secondary heating process, the lower the extrapolation starting temperature, and the lower the attenuation rate. tendency. In particular, it was confirmed that under the condition that the second temperature in the secondary heating process is 125°C, the extrapolation starting temperature is very high above 110°C, and the attenuation rate is very low, less than 10%. Furthermore, it was confirmed that in Examples 9 and 10, the attenuation rate can be significantly reduced by processing at a high temperature of 135°C in the second relaxation process.

As described above, according to the above-mentioned embodiment, the following effects are obtained. (1) During the processing of the piezoelectric film, even if the piezoelectric film is heated, the decrease in the piezoelectric constant d31 due to the processing can be suppressed. (2) Since the attenuation rate of the piezoelectric constant d31 can be ensured to be 20% or less, it is possible to more reliably suppress the processing caused by the composition of the piezoelectric film specified by [Condition 1] [Condition 2] The piezoelectric constant d31 is reduced. (3) The fluororesin that composes the piezoelectric film is a homopolymer of vinylidene fluoride. Compared with the structure of a copolymer of vinylidene fluoride, it is easier to increase the piezoelectric constant d31 of the piezoelectric film. (4) If the first temperature and the second temperature are equal to each other, the equipment used for heating at the first temperature and the equipment used for heating at the second temperature can also be shared.

In addition, the above-mentioned embodiment can also be changed and implemented as follows. ･ In the first relaxation process, different first temperatures can also be used, and separate heating methods are used at each first temperature. In addition, in the first relaxation process, mutually equal first temperatures may also be used, and a separate heating method may be used at each first temperature. ･ In the secondary heating process, different second temperatures can also be used. At each second temperature, separate heating methods are used. In addition, in the secondary heating process, second temperatures that are equal to each other can also be used, and separate heating methods are used at each second temperature. ･ Between the relaxation process and the secondary heating process, the storage form of the crystalline polymer film at room temperature is not limited to the storage in the state of being wound into a roll. For example, it can be set on a predetermined placement surface Keep it in a state where it is stacked on top. ･ In the second mitigation process, it is also possible to use different heating temperatures at a temperature higher than 115℃ and below 150℃. At each heating temperature, use separate heating methods and separate heating times. In addition, in the second mitigation process, it is also possible to use mutually equal heating temperatures at a temperature higher than 115°C and less than 150°C, and use separate heating methods and separate heating times at each heating temperature.

[Fig. 1] is a graph showing the relationship between the extrapolation starting temperature and the piezoelectric constant d31 in each example and comparative example.

Claims (6)

A piezoelectric film containing a fluororesin as a piezoelectric material, the fluororesin contains a repeating unit derived from vinylidene fluoride as a main structural unit, and the piezoelectric constant d31 of the piezoelectric film is 20pC/N or more, and The extrapolation starting temperature of shrinkage obtained by TMA measurement is 90°C or more and 135°C or less, and the piezoelectric film is heated at 100°C for 24 hours as a test treatment. The difference in the measured piezoelectric constant d31 is 20% or less with respect to the piezoelectric constant d31 before the aforementioned test treatment. The piezoelectric film described in the first item of the patent application, wherein the aforementioned fluororesin is a homopolymer of vinylidene fluoride. A method for manufacturing a piezoelectric thin film includes: a thin film forming process, stretching and polarization processing a sheet formed of a fluororesin, thereby forming a crystalline polymer thin film with piezoelectricity. The fluororesin includes Repeating units derived from vinylidene fluoride as the main structural unit; to ease the process, 90°C or more and 115°C or less is the first temperature, and the aforementioned crystalline polymer film is heated at the aforementioned first temperature for 5 seconds or more and 130 seconds or less By this, the crystalline polymer film is thermally fixed and the residual strain is relieved; and a secondary heating process is used to heat the crystalline polymer film after the relaxation process at the first temperature or higher and 140°C or lower. The piezoelectric film is manufactured by heating for 15 seconds or more and 120 seconds or less. The manufacturing method of piezoelectric thin film as described in item 3 of the scope of patent application, wherein In the secondary heating process, the crystalline polymer film is heated at the first temperature. A method for manufacturing a piezoelectric film, which is a method of manufacturing a piezoelectric film from a crystalline polymer sheet, and includes: a film forming process, stretching and polarization processing the sheet formed of fluororesin, By this, a crystalline polymer film with piezoelectricity is formed. The fluororesin contains repeating units derived from vinylidene fluoride as the main structural unit; and the process is relaxed, and the aforementioned fluororesin is heated at a temperature higher than 115°C and below 150°C. The crystalline polymer film is heated for 10 seconds or more and 140 seconds or less to thermally fix the crystalline polymer film and alleviate residual strain. A piezoelectric element is provided with the piezoelectric thin film as described in item 1 or 2 of the scope of patent application.

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